bump version, prepare to release

NEWS

+Version 12.3
+    - fixed setting webkit properties and user defined style sheet in html dialog
+    - improve stdin handler for html dialog
+    - update copyright notice
+
 Version 12.2
     - add --auto-scroll option as an alias to --tail
     - fixed jump to anchor in simple mode of html dialog

configure.ac

-AC_INIT([YAD], [12.2], [https://github.com/v1cont/yad/issues/], yad, [https://github.com/v1cont/yad/])
+AC_INIT([YAD], [12.3], [https://github.com/v1cont/yad/issues/], yad, [https://github.com/v1cont/yad/])
 AC_CONFIG_AUX_DIR([build])
 
 AM_INIT_AUTOMAKE([1.11 foreign dist-xz no-dist-gzip])